Minor Hysteresis Loop Testing for Magnetic Materials

Minor hysteresis loop testing, also referred to as local loop testing, is a critical process for evaluating the magnetic properties of materials operating under non-saturated conditions. This technique provides essential data regarding permeability and core loss at specific operating points, which is vital for the design of efficient electromagnetic components such as transformers and inductors.

Technical Overview

Minor hysteresis loop testing characterizes the magnetic behavior of materials when the applied magnetic field is insufficient to drive the material to saturation. Unlike the major hysteresis loop, which represents the maximum boundary of magnetic properties, the minor loop traces a smaller path within the B-H curve plane. This testing is essential because most electrical devices operate within specific excitation ranges rather than at full saturation. The shape and area of the local hysteresis loop directly influence parameters such as effective permeability and hysteresis losses at the designated operating point.

Testing Samples

Soft Magnetic Cores

Toroidal cores made of ferrite, nanocrystalline, or amorphous materials are the most common samples. These closed-loop geometries minimize demagnetizing effects and provide accurate magnetic property measurements.

Electrical Steel Sheets

Grain-oriented and non-oriented electrical steel strips are tested using standardized frames like the Epstein frame or Single Sheet Testers (SST). These samples are fundamental for transformer and motor core manufacturing.

Soft Magnetic Composites (SMC)

Compacted powder cores are tested to evaluate their localized magnetic performance, particularly under DC bias conditions or specific AC excitation levels.

Testing Items

The testing process quantifies several specific magnetic parameters that define the local behavior of the material:

• Amplitude Permeability: The permeability calculated at specific peak values of the magnetic flux density.
• Minor Loop Coercivity: The coercive force measured from the local loop, which often differs from the saturation coercivity.
• Remanence Flux Density: The residual magnetization when the local field is removed.
• Local Hysteresis Loss: The energy loss per cycle represented by the area enclosed by the minor loop.
• Incremental Permeability: The slope of the minor loop measured under a superimposed DC bias field.

Testing Methods

The standard approach for local hysteresis loop testing involves using a hysteresisgraph or a B-H analyzer. The test typically follows methodologies outlined in IEC 60404 standards or ASTM A341 specifications.

• Sample Preparation: The specimen is demagnetized prior to testing to ensure a neutral magnetic starting state.
• Excitation Control: A controlled current is applied to the primary winding to generate a specific magnetic field strength (H). The amplitude is carefully limited to prevent saturation.
• Measurement: Induced voltage in the secondary winding is integrated to calculate magnetic flux density (B). The system simultaneously records B and H values.
• Loop Tracing: The field is cycled symmetrically or asymmetrically around a specific point to trace the local hysteresis loop.

Applications

Minor hysteresis loop testing is crucial for industries where precision magnetic design is required:

• Power Electronics: Designing inductors and transformers for switch-mode power supplies where core materials operate at specific flux swings.
• Automotive Industry: Characterizing motor cores for electric vehicles to optimize efficiency under partial load conditions.
• Sensor Technology: Developing magnetic sensors that utilize the local slope of the B-H curve for signal detection.
• Material Science: Researching the Barkhausen effect and domain wall dynamics in ferromagnetic materials.

FAQ

How does a minor loop differ from a major loop?

A major loop is formed when the magnetic material is driven to complete saturation in both positive and negative directions. A minor loop is formed when the maximum applied field is below the saturation threshold, or when the material is cycled around a DC bias point. The permeability and losses derived from a minor loop are more representative of actual operating conditions in many electrical devices.

Why is demagnetization important before testing?

Demagnetization is essential to erase the magnetic history of the sample. If a sample retains a previous magnetization, the starting point for the minor loop test will be offset, leading to inaccurate measurements of permeability and coercivity. A clean magnetic state ensures the minor loop is symmetrical and traceable.

What standards apply to this testing?

Common standards include the IEC 60404 series, specifically parts regarding the measurement of magnetic properties of soft magnetic materials. ASTM A341 and ASTM A804 also provide guidelines for DC and AC magnetic testing methods used to generate hysteresis loops.